Traction on hard acceleration

[Daniel in SD]

Honestly, Daniel, I prefer to rely on on my own thinking and training.

I do too! I'm trying my best to explain my thinking.

1.It takes significantly more hp to accelerate at the same rate from 40 mph than it does from 0 mph due to air and mechanical rolling resistance, enough to make a difference on when you can chirp the tires or not.

That is absolutely correct. Technically it takes approximately 0hp accelerate from 0mph. HP = Torque x RPM ÷ 5252. So it takes twice as much horsepower to generate the same torque at twice the speed. I was talking about the torque required to spin the tires not horsepower. Torque is proportional to the force at the wheels. It takes the same force at the wheels to get them to slip no matter the speed. You can observe this from braking times and distances. It takes the same amount of time go from 30-0mph as it does to go from 60-30mph (though three times the distance). So if you're not spinning the tires with the torque you produce at 40mph the same torque at 0mph will also not spin the tires. Even if the aerodynamic drag were significant (which it clearly can't be given the Tesla's range) it would not change this. Applying a a force to the front of the car does not change how much traction it has.

2. And this is the point of my posting in this thread - the power modules on the basic S models use lower rated and less efficient CMOS inverters, and hence take a moment to deliver full torque to the wheels when you punch the throttle unlike the P models. Likely the same for the Model 3, which could explain less wheel spin.

On this I think we agree. The P models produce more torque/hp. The Model 3 does not produce enough torque to the wheels to spin the tires. The original poster's question was, why? Which is a good question given competitors cars with similar 0-60 times can do it easily. I've been trying my best to answer that question.

 

[Ben D]

The Model 3 does not produce enough torque to the wheels to spin the tires. The original poster's question was, why?

short answer is " because Tesla engineers decided it doesn't need to"

The controllers are also likely more than capable of supplying the electric motor with enough instantaneous amps to generate the torque needed to break traction at at standstill or at low speed, but the model 3 has been engineered for the masses. The masses driving an EV for the first time would probably freak out at the instantaneous torque available if the drivetrain was completely unleashed. Hence the controllers on these cars have been tuned to deliver ample acceleration sufficent for most purposes, but not enough to fry the tyres. Something that could be easily fixed with a software update, if they wanted to, but I suspect you will either have to disable traction control overrides or wait for the P versions if you want more than they have decided to provide at this early stage. Also , unlike ICE, traction control can work so fast on e-motors torque can be reduced instantaneously once excessive wheel slip has been detected by the wheel sensors. The engineers simply have decided to chose a consrvative wheel slip target. I assume a disable of the traction control/ABS would be the quickest fix (but wouldn't profess to know how to do it). But it looks like road and track did..

..
Tesla Model 3 Test Drive, Review - First Drive With the New Tesla Model 3

 

[Daniel in SD]

I'd be curious for someone with more direct knowledge of motor controllers to chime in. I was under the impression that sustained torque was limited by the maximum output current of the motor controller and the maximum power is limited by the maximum voltage and maximum current. This would be the reason for the flat torque and flat horsepower regions of the curve. I'm sure Tesla could implement some sort of quick "overboost" feature to break the tires loose.
I don't think the acceleration is being limited by traction control. There's a light for traction control isn't there? Also people are starting to put much stickier and wider tires on their cars. I haven't heard anyone say the 0-60 is improved.

 

[Daniel in SD]

Looks like they did do a donut! I wonder if they were able to defeat the traction control or maybe the slip start button does? It takes much less power to spin the wheels when doing a donut than it does in a straight line. You can see that the right wheel isn't even spinning so all the torque is going to one wheel. The Model 3 has an open rear differential.

 

[silentsnow31802]

Looks like they did do a donut! I wonder if they were able to defeat the traction control or maybe the slip start button does? It takes much less power the spin the wheels when doing a donut than it does in a straight line. You can see that the right wheel isn't even spinning so all the torque is going to one wheel. The Model 3 has an open rear differential.

I can tell you for sure slip start does not allow that much wheel spin. Ive always though the 3 has some form of LSD. Every time I try to spin the tires I get both not just 1. I am turning quite hard most the time though so maybe its just sliding.

 

[Daniel in SD]

I can tell you for sure slip start does not allow that much wheel spin. Ive always though the 3 has some form of LSD. Every time I try to spin the tires I get both not just 1. I am turning quite hard most the time though so maybe its just sliding.

Like most cars these days it probably does apply the brakes to the slipping wheel to simulate an LSD. Real LSDs cost money and are unnecessary for street driving. In that Road and Track video one tire is just sliding wheel the other is spinning. The new roadster gets rid of the differential and has two motors in the rear for torque vectoring. That would be a nice option to have on a performance version of the 3. Too bad Tesla says they'll void your warranty if you track or autocross the Model 3.

 

[Krash]

The raw runlogs are here.

There are five primary limits on acceleration performance. Chronologically from launch on a full SoC they are:

1. Launch mode - only software enabled on Performane Models
2. Torque limit - Teslas have a fixed torque setting which is very conservatively set on the new drive units. P models are set higher. Determines performance from launch umtil max power is reached.
3. Max Power Limit - Differentiated by models but also limited by voltage and current as seen by the 75D with rare BTX8 (P85) battery. This is why the 100D has better high speed passing than the 75D.
4. Back EMF - Differs by battery current and maybe voltage and likely different for the large drive unit. Starts to limit power at about 70mph. Not a Tesla setting per se but a design limitation of the motors.
5. Top Speed Limiter - Different on P model but related to RPM limits of motors at given gear ratios.

 

[Bricepark]

I found two YouTube videos of Model 3 dynamometer tests. One yielded almost 400 HP which I do not believe is accurate. The other yielded 281 HP at the wheels and 552 ft. lb. torque. Testing required disabling the front wheel speed sensor(s). I suspect Tesla uses wheel speed data to determine if one wheel is faster than the others and uses that input to regulate motor output. A mechanical solution would seem inelegant for an electronically controlled drive train.

 

[Daniel in SD]

I found two YouTube videos of Model 3 dynamometer tests. One yielded almost 400 HP which I do not believe is accurate. The other yielded 281 HP at the wheels and 552 ft. lb. torque. Testing required disabling the front wheel speed sensor(s). I suspect Tesla uses wheel speed data to determine if one wheel is faster than the others and uses that input to regulate motor output. A mechanical solution would seem inelegant for an electronically controlled drive train.

The front wheels need speed sensors because there is no drivetrain in the front. They are used for ABS, traction control, and stability control. The back wheels individual speed sensors because while the motor controller knows the speed of the motor it does not know the speed of the individual rear wheels.

 

[mattack4000]

The question wasn’t really about peak power or power curve, I just want to know how tesla is able to avoid the wheel spin off the line.

 

[Bricepark]

I did not read the response from Krash before posting mine. He is far better informed and I appreciate him sharing his knowledge.

 

[Skidmark]

The question wasn’t really about peak power or power curve, I just want to know how tesla is able to avoid the wheel spin off the line.

Sofar it seems like the controller is limiting the amount of power available at lower speeds so it's not possible to spin the wheels in normal dry driving conditions and relatively warmer temperatures.

I had a 2014 S85 loaner once and could not spin the wheels from a stop even in the wet. This was after disabling traction control (it was possible to disable TC on this car, but not in my 2016 S 90D). I did get it to break loose while punching it hard while cornering...that was fun.

 

[mattack4000]

I think you are right, else rwd cars will just sit and do burn out

 

[12Pack]

Sofar it seems like the controller is limiting the amount of power available at lower speeds so it's not possible to spin the wheels in normal dry driving conditions and relatively warmer temperatures.

I had a 2014 S85 loaner once and could not spin the wheels from a stop even in the wet. This was after disabling traction control (it was possible to disable TC on this car, but not in my 2016 S 90D). I did get it to break loose while punching it hard while cornering...that was fun.

Agree - that was also the point that a couple of us were making earlier in the thread. In spite of the theoretically depicted full torque available from 0 rpm from the motor - the non-P models do not in fact have that available at the wheels immediately from standstill.

 

[Daniel in SD]

Agree - that was also the point that a couple of us were making earlier in the thread. In spite of the theoretically depicted full torque available from 0 rpm from the motor - the non-P models do not in fact have that available at the wheels immediately from standstill.

I don’t see that from that acceleration plot. It looks like full torque is available at very low speeds. Also, it takes the same amount of torque to skid the tires no matter what the speed. Torque just translates to force at the point where the tire touches the road.

 

[Uncle Paul]

I believe that Tesla has implemented traction control by comparing the speed of the front wheels to the speed of the rear wheels. It the rear wheels are spinning faster, the motor torque is reduced to stop the spinning. Only enough torque is provided to optimize forward motion with minimal spinning.

Not as much fun, but certaintly safer.

 

[Daniel in SD]

I believe that Tesla has implemented traction control by comparing the speed of the front wheels to the speed of the rear wheels. It the rear wheels are spinning faster, the motor torque is reduced to stop the spinning. Only enough torque is provided to optimize forward motion with minimal spinning.

Not as much fun, but certaintly safer.

This is certainly true but isn’t there a light that shows the traction control is working? It doesn’t seem to me that traction would be limited by the tires for a 2.2 sec 0-30mph time. Plenty of RWD cars are much faster than that. I guess we’ll see if anyone can get better times with stickier tires.

 

[Skidmark]

I don't have my 3 yet, but when I get it, I'll also get a GTech acceleration log from a standstill. I've felt TC kick in on my S 90D when I punched it at ~25mph on a colder night in SF, and it's very noticeable, causing a shock to the car that's similar to hitting a pothole. I very much doubt that TC is what's preventing wheelspin on a Model 3 in normal weather/temperature conditions. Although TC on an EV can work much more quickly than on an ICE car, it doesn't have the ability to completely prevent any wheelspin, so the obvious answer is that the there isn't enough force to cause wheelspin.

 

[Daniel in SD]

I don't have my 3 yet, but when I get it, I'll also get a GTech acceleration log from a standstill. I've felt TC kick in on my S 90D when I punched it at ~25mph on a colder night in SF, and it's very noticeable, causing a shock to the car that's similar to hitting a pothole. I very much doubt that TC is what's preventing wheelspin on a Model 3 in normal weather/temperature conditions. Although TC on an EV can work much more quickly than on an ICE car, it doesn't have the ability to completely prevent any wheelspin, so the obvious answer is that the there isn't enough force to cause wheelspin.

Awesome. I look forward to seeing the data. I'm still curious why everyone seems to think that the torque spikes up at around 30mph.
On the other hand I've driven a Spark EV and the traction control is subtle enough I could see someone not noticing it so it's possible the Model 3 has a more sophisticated TC system than the Model S.

 

[Krash]

I would use the term "fixed torque" rather than "full torque" as the drive unit is capable of higher torque than the performance settings.

As for traction control, keep in mind that on a straight, dry and smooth surface the fixed torque settings are low enough that the tires will not spin. I assume the staggered wheel setup on the performance models was implemented to put slightly more rubber on the road to prevent spin at those settings above 900Nm. This is a very different mechanism than what most think of as traditional traction control that limits spin on surfaces like gravel or ice.